The present invention relates generally to inspection of integrated circuit devices, and, more particularly, to a method for preparing a specimen for application of microanalysis, such as transmission electron microscopy (TEM).
Advancements in Transmission Electron Microscopy, or TEM, technology enables materials to be analyzed at near atomic resolution by providing high-magnification, high-resolution imaging and analysis capabilities. TEM enables scientists to gather information relating to a material's physical properties, such as its microstructure, crystalline orientation and elemental composition. This information has become increasingly important as the need for advanced materials for use in areas such as microelectronics and optoelectronics, biomedical technology, aerospace, transportation systems and alternative energy sources, among others, increases.
TEM is accomplished by examining material specimens under a transmission electron microscope. In a transmission electron microscope, a series of electromagnetic lenses direct and focus an accelerated beam of electrons, emitted from an electron gun contained within the microscope, at the surface of a specimen. Electrons transmitted through the specimen yield an image of the specimen's structure, which provides information regarding its properties. In addition, elemental and chemical information is provided by both the transmitted electrons and the x-rays that are emitted from the specimen's surface as a result of electron interaction with the specimen. Thus, because it is necessary for the electron beam to transmit through the specimen, a key component of successful material analysis by TEM techniques is the condition and preparation of the specimen itself.
Before a specimen can be analyzed using TEM, it must be prepared using various techniques to achieve the necessary electron transparency. This electron transparency is accomplished by thinning a defined area of the specimen. For equal resolution, the required thickness of the specimen is dependent on the accelerating voltage of the transmission electron microscope. For example, using a 120 kV microscope, the specimen thickness must be on the order of about 100 to about 2000 angstroms (Å). In contrast, A 1,000 kV microscope can tolerate a specimen thickness of up to about 5,000 Å.
Specimens are prepared through several well-known methods, including, but not limited to, electrolytic thinning, mechanical grinding, ultramicrotomy, crushing, and ion milling. Often times, multiple methods are utilized to prepare a single specimen. For most types of specimens, either electrolytic thinning or ion milling is used as the final form of specimen preparation. In both cases, amorphous damage ranging in thickness from 1–10 nanometers may result, particularly in the case of ion milling. In this case, the energy of the ion beam transforms the crystalline structure of the material to an amorphous state. This amorphous damage adversely affects the quality of the TEM analysis because it alters the natural characteristics of the material.
Accordingly, one way of protecting a specimen (such as a resist covered substrate) is to deposit a conductive metal layer (e.g., platinum, tungsten, gold, copper, aluminum, titanium, etc.) over the surface by a physical vapor deposition process to release charges from the TEM microscope electron beam bombardment or a focused ion beam etching process. In particular, platinum is a preferred metal because it is a stable metal that can be formed at a very thin thickness. Metal deposition on a selected area is advantageous in that it results in high throughput and lower costs. In addition, the entire wafer is not impacted because of the localized coating, and can be subsequently used in production or development. Moreover, a typically applied process of ion beam initiated metal encapsulation can cause edge roundness, flat top edge rounding and blurred boundary definition, which raises a serious issue if TEM is used to place emphasis on the feature topography, such as true feature boundary definition for critical dimension (CD) measurement. The critical dimension measurement can in turn qualify the TEM as a high-resolution metrology reference system for semiconductor metrology.